ANDROID: usb: gadget: f_mtp: Return error if count is negative

If the user passes in a negative file size in a int64,
this will compare to be smaller than buffer length,
and it will get truncated to form a read length that
is larger than the buffer length.

To fix, return -EINVAL if the count argument is negative,
so the loop will never happen.

Bug: 37429972
Bug: 161328074
(cherry-picked from commit 6ba119257516c87a577993487c0b5aaa1ab0c0a1)
Signed-off-by: rickyniu <rickyniu@google.com>
Change-Id: I8f055b0186931f1ebd222bc17bbfd96a7aedd459